Pharo by Example vol 2: new chapter available

> On Mar 4, 2010, at 11:38 AM, Henrik Johansen wrote:
>
>  
>> FWIW, this quote from the following discussion on Squeak-dev distills what I was trying (but failing) to demonstrate:
>> "We shouldn't stop trying to support well-behaved ensure blocks properly.
>> Because if we don't then ensure
>> becomes entirely random. Or can anyone here explain why it would be
>> reasonable to expect that this version works:
>>
>> p := [[Processor yield] ensure:[Transcript show: 'finished']] fork.
>> Processor yield.
>> p terminate.
>>
>> but this one doesn't:
>>
>> p := [[] ensure:[Processor yield. Transcript show: 'finished']] fork.
>> Processor yield.
>> p terminate.
>>
>> (we're using yield here to denote any kind of process switch) If the
>> only thing that determines whether an ensure block is executed is a
>> process switch, then we basically have random, non-predictable unwind
>> handling. Which is a problem that is entirely unrelated to whether the
>> unwind block is itself well-behaved or not. "
>>
>> In a book with chapters discussing topics such as ensure and processes, (based on Pharo 1.0 which is unlikely to contain any fixes for such an issue), it deserves at least a mention.
>>
>> Cheers,
>> Henry
>>
>> Den 03.03.2010 15:33, skrev Levente Uzonyi:
>>    
>>> On Wed, 3 Mar 2010, Stéphane Ducasse wrote:
>>>
>>>      
>>>> On Mar 2, 2010, at 10:33 PM, Levente Uzonyi wrote:
>>>>
>>>>        
>>>>> On Tue, 2 Mar 2010, Henrik Sperre Johansen wrote:
>>>>>
>>>>>          
>>>>>>            
>>>>>>>> PS. For the not-so-faint-of-heart, open a Transcript and try evaluating:
>>>>>>>> |proc|
>>>>>>>> proc := [[Transcript show: 'Start!'. ] ensure: [Transcript show:
>>>>>>>> 'Finish!' Processor yield.]] newProcess.
>>>>>>>> proc resume.
>>>>>>>> Processor yield.
>>>>>>>> proc suspend.
>>>>>>>> proc terminate.
>>>>>>>>                
>>>>>>> Why don't we get it printing?
>>>>>>>              
>>>>>> Forgot a . there, supposed to be
>>>>>>
>>>>>> |proc|
>>>>>> proc := [[Transcript show: 'Start!'. ] ensure: [Transcript show:
>>>>>> 'Finish!'. Processor yield.]] newProcess.
>>>>>> proc resume.
>>>>>> Processor yield.
>>>>>> proc suspend.
>>>>>> proc terminate.
>>>>>>
>>>>>> on my machine it prints:
>>>>>> *Start!Start!Finish!
>>>>>>            
>>>>> The problem occurs because Transcript >> #endEntry (sent from #show:)
>>>>> takes a while to execute, so the process (proc) prints 'Start!', but
>>>>> it gets terminated before execution reaches #resetContents (#reset in
>>>>> Squeak). So 'Start!' is still in the stream. Then our process executes the
>>>>> #ensure: block and it prints 'Start!' and 'Finish!' too.
>>>>>          
>>>>
>>>> Thanks for the explanation levente!
>>>> But I'm still dizzy :) why Transcript show: 'Finish' would also print 'start' ?
>>>>        
>>> Transcript is just a stream. #show: is just a combination of #nextPutAll: and #endEntry. #endEntry displays the contents of the stream and when that's done it removes everything from the stream with #resetContents. #resetContents is never executed, because the process is terminated before that. So the contents of the stream are kept and displayed again later.
>>>
>>>
>>> Levente
>>>
>>>      
>>>>        
>>>>> There's worse problem with #ensure: and #terminate is that, a process executing an #ensure: block can be terminated. If you evaluate this code:
>>>>>
>>>>> | process stage1 stage2 stage3 counter |
>>>>> stage1 := stage2 := stage3 := false.
>>>>> counter := 0.
>>>>> process := [
>>>>>     [ stage1 := true ] ensure: [
>>>>>         stage2 := true.
>>>>>         1000000 timesRepeat: [ counter := counter + 1 ].
>>>>>         stage3 := true ] ] newProcess.
>>>>> process resume.
>>>>> Processor yield.
>>>>> process suspend.
>>>>> process terminate.
>>>>> 1000 milliSeconds asDelay wait.
>>>>> { stage1. stage2. stage3. counter } explore
>>>>>
>>>>> you will find that stage1 and stage2 is reached as expected, but stage3 is not and counter is less than 1000000. That's because the forked process started evaluating the #ensure: block, but it was terminated by our process.
>>>>>
>>>>> Is this the expected behavior when sending #terminate to a process which is evaluating an #ensure: block?
>>>>>
>>>>>
>>>>> Cheers,
>>>>> Balázs & Levente
>>>>>
>>>>>
>>>>>          
>>>>>> *yay
>>>>>> Henry*
>>>>>> *
>>>>>>            
>>>>>          
>>>>
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